#2 Shinsuke Nakamura

Now this one is just mind-boggling. Shinsuke Nakamura signing with WWE in 2016 was a major deal. The singing announcement was treated as a big deal, his arrival in NXT was a big deal and his run in NXT was a big deal. Nakamura was arguably the biggest star in all of wrestling in Japan at the time of his signing and brought all his hype, skill and mystique with him.
Nakamura had a very successful run in NXT, where he was a 2-time NXT Champion and defeated everybody in his way. His main roster debut was handled pretty well for the most part, as he was generally featured in the main event scene for most of 2017. Nakamura would then win the best WWE Royal Rumble match in nearly a decade, and go onto face AJ Styles at WrestleMania. Here is where things went downhill.
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Nakamura would have a very misguided heel turn at WrestleMania and would fall right down the card following his feud with Styles. Due to him having to use so many heel tactics, particularly the low blow, Nakamura's matches have also dipped in quality. Ever since the summer, Nakamura has barely been featured in any feuds and has been treading water in a go-nowhere tag team with Rusev since January.
Nakamura reportedly does plan on sticking with WWE but has apparently 'mentally checked out' and is just going through the motions in the ring, while he enjoys life outside the ring with his family. Regardless of that all being true or not, I think it's time WWE either changes him back to the way he was pre-heel turn, along with a push, or Nakamura should seriously consider joining All Elite Wrestling, where he would shine and be treated like the star he is.